<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        div {
            width: 200px;
            height: 200px;
            margin: 100px;
            background-color: aquamarine;
        }
    </style>
</head>
<body>
    <div>
    </div>
    <script>
        var div = document.querySelector('div');
        div.onmousemove = function(e) {
            var div_left = div.offsetLeft;
            var div_top = div.offsetTop;
            var left = e.pageX - div_left;
            var top = e.pageY - div_top;
            div.innerHTML = 'x:' + left + ',y:' + top
        }
    </script>
</body>
</html>